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L'objectif de cette thèse est le contrôle, par un champ magnétique, de la structure interne d'une paroi de domaines. Nous avons sélectionné le cas d'une paroi de<br />Bloch asymétrique, que nous avons étudié dans un système modèle : des plots épitaxiés autoassemblés de Fe(110). Nous avons mis en évidence la possibilité de procéder au retournement accompagné d'un phénomène d'hystérésis magnétique d'un des degrés de liberté internes, le sens des domaines de fermeture de Néel qui terminent la paroi à chaque surface. Ceci a été observé à rémanence après aimantation et de manière statistique par microscopie magnétique XMCD-PEEM, en accord quantitatif avec des simulations micromagnétiques. Des résultats préliminaires de la mise en évidence directe sous champ magnétique ont été obtenus par microscopie de Lorentz. Une étude sous champ et en température a été menée par magnéto-transport sur des plots individuels. Pour ce faire, un procédé de contactage de plots individuels par lithographie électronique a été développé. En complément mais sur le même système expérimental, nous avons étudié comment s'opère la transition entre une paroi magnétique (2D) et un vortex magnétique (0D), que nous avons montré être de second ordre. La transition et les fluctuations stochastiques entre une paroi et un vortex ont été mises en évidence par microscopie de Lorentz.

SiGe nanostructures on crystalline Si substrates with (001) orientation are among the most studied system in condensed matter physics and nanoscience. This interest has been mainly driven by the important potential applications in opto and nano-electronic devices thanks to the improvement of the optical and electronic properties compared to bulk systems. These features come essentially from the possibility of engineering the strain field within the nanostructures using the lattice mismatch of ~ 4 % between Ge and Si and from the spatial confinement, capable of modifying the electronic band structure leading to an increase of the charge carrier mobility. It is obvious that these applications largely depend on the control of surface processes during the growth of the nanostructures, and their performance are strongly dependent on strain relaxation and dislocation injection. Besides the technological interest, the SiGe/Si(001) system has received much attention since it is also a model for understanding the fundamental processes occurring during 3D island formation and self-organization phenomena. In fact, the lattice mismatch between Ge and Si introduces a stress field which has dramatic effects on the growth process and is responsible for a number of structural and electronic phenomena. In particular, the stored elastic energy can be partially relieved by spontaneous formation of 3D objects of nanometric size on top of a pseudomorphic SiGe wetting layer. This growth mode, called Stranski-Krastanov (SK), is a way of easily forming self-assembled nanostructures, which can be used to obtain quantum confinement of charge carriers in nanoelectronics device applications. In recent years, considerable efforts have been devoted to the growth of hetero-epitaxial SiGe nanostructures with well controlled size, shape and positioning, and with defined stoichiometry and strain state. However, some aspects still need to be addressed for a complete understanding of this system, including: (i) the competition between kinetic and thermodynamic factors for island formation, (ii) the mechanisms governing the relative growth of individual nanostructures, (iii) the interplay between SiGe intermixing and strain relaxation mechanisms. In the present work, we carry out an experimental investigation of the relationship between morphology, elemental composition, strain state and electronic structure of self-assembled and lithographically defined SiGe nanostructures by means of several spectro-microscopy techniques. The Si and Ge diffusion dynamics and the self-organization phenomena during the growth of SiGe islands have been studied by Scanning Auger Microscopy (SAM) and Atomic Force Microscopy (AFM). Micro-Raman, SAM and Scanning Transmission Electron Microscopy coupled with Electron Energy Loss Spectroscopy (STEM-EELS) techniques have been used for the investigation of the interplay between strain relaxation mechanisms and SiGe intermixing in self-assembled islands. The effects of strain and composition on the electronic band structure in lithographically defined SiGe nanostructures, in layout very close to those used in prototype devices, have been characterized with nanoscale spatial resolution joining information from Tip Enhanced Raman Spectroscopy (TERS), nanofocused X-Ray Diffraction (XRD) and Energy-Filtered PhotoElectron Emission Microscopy (PEEM). The thesis is conceptually divided in two main parts: the first, to which belong Chapters 1, 2 and 3, deals with the experimental investigation of the Ge surface diffusion and of the self-organization phenomena of SiGe islands grown in a bottom-up approach; the second, including Chapters 4 and 5, is based on the experimental characterization of the strain state and of the strain-induced effects on the electronic band structure of lithographically defined SiGe nanostructures obtained in a top-down approach Chapter 1 presents an overview on the basic processes occurring during hetero-epitaxial growth of thin solid films. In the Chapter 2 the surface diffusion of Ge on a clean and C covered Si(001) surface promoted by annealing at high temperatures in UHV of pure Ge stripes is experimentally investigated by means of in-situ Scanning Auger Microscopy. The influence of a controlled carbon coverage on the Ge surface diffusion is quantitatively studied, showing that the diffusion coefficient presents a strong dependence on carbon coverage (see Fig. 1(a)). Chapter 3 deals with the experimental investigation of the growth process of self-assembled SiGe islands on Si(001) (see Fig. 1(b)). From the size and density evolution exhibited by the nucleated islands, we propose a scenario where the island growth is essentially driven by kinetic factors within a diffusion limited regime. Finally, we investigated the interplay among SiGe intermixing and plastic relaxation, showing that the surface thermal diffusion growth method leads to the formation of coherent islands (dislocation-free), as shown in Fig. 1(c), larger than those attainable by MBE and CVD. Chapter 4 presents the mapping with nanoscale resolution of strain, composition, local work function and valence band structure of lithographically defined SiGe embedded nano-stripes using TERS and Energy-Filtered PEEM (see Fig. 1(d) showing the Ge concentration mapping of the nano-stripes as obtained by PEEM analysis). In Chapter 5 are presented the first results of a direct characterization of the strain state of lithographically defined SiGe nano-ridges using the recently developed nanofocused XRD technique. The work presented in this thesis is the outcome of an experimental PhD research project developed at the Politecnico di Milano (Milano, Italy) in co-tutorship with the École Polytechnique (Paris, France) and the French Atomic Energy Commission (CEA-Saclay, France). SAM and AFM have been performed at Department of Physics of the Politecnico di Milano. Micro-Raman Spectroscopy has been carried out at the Materials Science Department of the Università Milano-Bicocca. PEEM measurements have been realized at CEA and during two standard experimental runs at the TEMPO beamline of SOLEIL Synchrotron (France). TERS and preliminary TEM analysis have been performed at the École Polytechnique, while more extensive TEM and STEM-EELS measurements have been developed at IMM-CNR in Catania. The nano-XRD experiment has been carried out during a standard experimental run at ID13 beamline of the European Synchrotron Radiation Facility (ESRF). The close collaboration with the laboratory L-NESS in Como made available the set of the lithographically-defined investigated samples. The experimental results have been exploited in close collaboration with a theory group at the Materials Science Department of the Università Milano-Bicocca for a deeper insight into the atomic level mechanisms during island growth process.

Dans ce document, j'utilise des résultats de ma recherche des dix dernières années pour montrer que le dichroïsme magnétique des rayons x est une technique puissante pour déterminer les propriétés magnétiques des couches minces, multicouches et nanostructures magnétiques. L'utilisation de règles de somme pour le dichroïsme permet de déterminer quantitativement les contributions orbitale et de spin au moment magnétique, et ceci séparément pour chaque élément dans un matériau hétérogène.<br />Utilisé de façon qualitative, le dichroïsme permet de suivre l'aimantation de différentes couches d'une multicouche en fonction du champ appliqué. En combinaison avec la structure temporelle du rayonnement synchrotron, il est possible d'étudier le renversement rapide de l'aimantation avec sélectivité chimique, ce qui est important pour les dispositifs comme les vannes de spin et les jonctions tunnel magnétiques. En ajoutant la résolution spatiale d'un microscope électronique comme le PEEM, il devient possible d'étudier tous les détails du renversement rapide dans les systèmes magnétiques complexes.

Diese Arbeit untersucht Fe-Oxid-Systeme mit Hilfe einer Kombination aus Mikroskopie (LEEM, Röntgen PEEMs), Beugung (LEED) und Spektroskopie (XPS) und berichtet über die elektronenoptische Entwicklung adaptiver Optiken und Aberrationskorrekturen für einen elektrostatischen abbildenden Energieanalysator. Experimentell untersuchten wir Magnetit und Hämatit Dünnschichten. Ihre Kristallstruktur, Stöchiometrie sowie deren Oberflächenterminierung können durch spezielle Herstellungsverfahren eingestellt werden. Unter Ausnutzung der Echtzeit-Beobachtung mit Mikroskopie, Beugung und Spektroskopie untersuchten wir (a) die Oberflächenmodifikationen von Fe3O4 und α-Fe2O3-Dünnschichten durch Fe Ablagerung; (b) die reversible Phasenumwandlung Fe3O4 ↔ α-Fe2O3 unter verschiedenen Oxidationsbedingungen; (c) die Bildung der metastabilen γ-Fe2O3-Phase und (d) die Wechselwirkung von Fe3O4 und α-Fe2O3 Oberflächen mit unterstützten Pt-Nanopartikeln. Es wurde ein Algorithmus entwickelt, um den LEEM Bildkontrast für inhomogene 2D Oberflächen zu simulieren. Abschließend wird das Design eines Energiefilter-System vorgestellt, das in ein PEEM/LEEM Mikroskop der neuen Generation eingebaut werden wird. Das System basiert auf dem gleichen Abbildungsprinzip wie der magnetische Ω-Filter, der erfolgreich im aktuellen SMART Mikroskop eingesetzt wird. Das neue Instrument zielt auf die Verbesserung der Orts- und Energieauflösung im XPEEM (5 nm und 70 meV). Die Mehrzahl der möglichen Aberrationen zweiter Ordnung wird durch die intrinsische Symmetrie selbstkompensiert. Die Wirkung der anderen Aberrationen wird durch ein geeignetes Design der Verzögerungs- und Beschleunigungsoptiken kombiniert mit einer optimierten Passenergie reduziert. Darüber hinaus kompensieren zusätzliche Hexapole die restlichen dominierenden Aberrationen, wodurch eine Orts- und Energieauflösung besser als 2 nm bzw. 75 meV erreicht wird. / This work presents the investigation of Fe-oxide systems, combining microscopy (LEEM, X-PEEM), diffraction (LEED) and spectroscopy (XPS), and the electron-optical development of adaptive optics and aberration corrections for an electrostatic imaging energy analyzer. Experimentally, we studied magnetite (Fe3O4) and hematite (α-Fe2O3) thin films. Their crystal structure, stoichiometry as well as their surface termination can be tuned by special preparation procedures. Taking advantage of real time observation with microscopy, diffraction and spectroscopy, we investigated (a) the surface modifications of Fe3O4 and α-Fe2O3 thin films by Fe deposition; (b) the reversible phase transformation Fe3O4 ↔ α-Fe2O3 under different oxidation conditions; (c) the formation of the metastable γ-Fe2O3 phase and (d) the interaction of Fe3O4 and α-Fe2O3 surfaces with supported Pt nanoparticles . An algorithm was developed to simulate the LEEM image contrast for inhomogeneous 2D surfaces. The possible application to experimental data and the limitation will be discussed. Finally, the design of an energy filtering system is presented, which will be implemented in a new generation PEEM/LEEM microscope. The system bases on the same imaging principle as the magnetic Ω-filter, successfully implemented in the actual SMART microscope. The new instrument aims for the improvement of lateral and energy resolution in X-PEEM (5 nm and 70 meV, respectively). The majority of the possible second order aberrations are self-compensated by intrinsic symmetry. The effect of the other aberrations is reduced by an adequate design for the deceleration-acceleration optics in combination with optimized pass energy. Furthermore, additional hexapole multipoles compensate for the residual dominating aberrations, yielding in the lateral resolution and energy resolution better than 2 nm and 75 meV, respectively.